1. Field of the Invention
This invention relates to multiplex addressing of ferro-electric liquid crystal displays. Such displays use a tilted chiral smectic C. I. or F liquid crystal material.
2. Discussion of Prior Art
Liquid crystal devices commonly comprise a thin layer of a liquid crystal material contained between two glass slides. Optically transparent electrodes are formed on the inner surface of both slides. When an electric voltage is applied to these electrodes the resulting electric field changes the molecular alignment of the liquid crystal molecules. The changes in molecular alignment are readily observable and form the basis for many types of liquid crystal display devices.
In ferro electric liquid crystal devices the molecules switch between two different alignment directions depending on the polarity of an applied electric field. These devices have a degree of bistability and tend to remain in one of the two switched states until switched to the other switched state. This allows the multiplex addressing of quite large displays.
One common multiplex display has display elements, ie pixels, arranged in an x, y matrix format for the display of e.g., alpha numeric characters. The matrix format is provided by forming the electrodes on one slide as a series of column electrodes, and the electrodes on the other slide as a series of row electrodes. The intersections between each column and row form addressable elements or pixels. Other matrix layout are known, e.g. polar co-ordinate (r-.theta.), and seven bar numeric displays.
There are many different multiplex addressing schemes. A common feature is application of a voltage, called a strobe voltage to each row or line in sequence. Coincidentially with the strobe applied at each row, appropriate voltages, called data voltages, are applied to all column electrodes. The differences between the different schemes lies in the shape of the strobe and data voltage waveforms.
European Patent Application 0,306,203 describes one multiplex addressing scheme for ferro electric liquid crystal displays. In this application the strobe is a unipolar pulse of alternating polarity, and the two data waveforms are rectangular waves of opposite sign. The strobe pulse width is one half the data waveform period. The combination of the strobe and the appropriate one of the data voltages provides a switching of the liquid crystal material.
Other addressing schemes are described in GB 2,146,473-A; GB-2,173,336A; GB-2,173,337-A; GB-2,173,629-A; WO 89/05025 ; Harada et al 1985 S.I.D Digest Paper 8.4 pp 131-134; and Lagerwall et al 1985 IEEE, IDRC pp 213-221; Proc 1988 IEEE, IDRC p 98-101 Fast Addressing for Ferro Electric LC Display Panels, P Maltese et al.
The scheme disclosed in GB -2,173,336A (EP 0197742) inventor Ayliffe, uses two data waveforms of rectangular shape and opposite sign with a period of two time slots (2ts). A strobe is a single pulse of one time slot duration and its is combined with the first half of a data waveform to cause selective switching in the first time slot. Prior to this selective switching each line is blanked by a blanking pulse. Such a blanking pulse or pulses may immediately precede the strobe so that a modified strobe waveform is formed. However, such a modified strobe waveform is merely a blanking immediately followed by a strobe pulse and it is the strobe pulse that combines with a selected data to cause selective switching.
The material may be switched between its two states by two strobe pulses of opposite sign, in conjunction with a data waveform. Alternatively, a blanking pulse may be used to switch the material into one state, and a single strobe pulse used with an appropriate data pulse to selectively switch back pixels to the other state. Periodically the sign of the blanking and the strobe pulses are alternated to maintain a net zero d.c. value.
These blanking pulses are normally greater in amplitude and length of application than the strobe pulses so that the material switches irrespective of which of the two data waveforms is applied to any one intersection. Blanking pulses may be applied on a line by line basis ahead of the strobe, or the whole display may be blanked at one time, or a group of lines may be simultaneously blanked.
One known blanking scheme uses blanking pulse of equal voltage (V) time (t) product Vt, but opposite polarity, to the strobe pulse Vt product. The blanking pulse has an amplitude of half and a time of application of twice that of the strobe pulse. These values ensure the blanking and strobe have a net zero d.c. value without periodic reversal of polarity. Experimental use has shown the scheme to have a poor performance.
Another known scheme with a blanking pulse is described in EP 0,378,293. This uses a conventional d.c. balanced strobe pulse (of equal periods of opposite polarity) with a similar d.c. balanced blanking pulse (of equal periods of opposite polarity) in which the width of the blanking pulse may be several times that of the strobe pulse. Such a scheme has a net zero d.c. value without periodic reversal of polarity of blanking and strobe waveforms.
The feature of d.c. balance is particularly important in projection displays since if it is desired to switch the gap between pixels to one optical state then periodic reversal of polarities is not permissible.
One problem with existing displays is the time taken to address complex displays. In order to drive complex displays at video frame rates it is necessary to address the display quickly. Contrast ratio can also be improved by addressing quickly so that the column waveform is at a correspondingly high frequency. However, merely increasing the speed of addressing will not always result in correct switching. An object of the present invention is to reduce the time taken to address a matrix display and to improve display contrast.